1. Field of the Invention
This invention relates to an image formation method and apparatus for forming an image by conductive developer on an image bearing member such as a photosensitive member.
2. Description of the Prior Art
As an electrophotograhic method for forming a toner image on a photosensitive member, a method is widely known in which the photosensitive member is uniformly charged, an original image light or an image light by a signal-modulated light beam is applied to the photosensitive member to thereby create an image pattern-like difference in charge density on the photosensitive member and form an electrostatic latent image thereon and this latent image is developed by an insulative or conductive dry developer. Where the insulative developer is used, other particles are mixed therewith and triboelectrification is effected on a developing sleeve or triboelectrification is effected by bringing the developer into contact with the developing sleeve and the developer is caused to adhere to the surface of the photosensitive member by a coulomb force acting between the charged developer and the latent image charge on the photosensitive member to thereby form a visualized image.
Where the conductive developer is used, a charge of the opposite polarity to the polarity of the latent image charge on the photosensitive member is induced in the developer existing on the grounded developing sleeve by the charge existing on the photosensitive member and the developer is caused to adhere to the surface of the photosensitive member by a coulomb force acting therebetween to thereby form a visualized image.
Where either of the insulative and conductive developers is used, so-called fog may sometimes be created in the background of the image, and this often occurs when the charge to be removed during image exposure remains on the photosensitive member. To eliminate such fog phenomenon, a positive or negative bias is in some cases applied to the developing sleeve to prevent any potential difference from being created between the white ground portion of the photosensitive member and the developer adhering to the developing sleeve during development. This image formation method will hereinafter be referred to as the first method.
There is also an image formation method in which an image light is pre-applied to a photosensitive member to form a conductive pattern thereon and a DC bias is applied between a developing sleeve and a conductive substrate provided on the photosensitive member, whereby a uniform potential difference is formed between conductive developer and the surface of the photosensitive member and a visualized image by the developer corresponding to said conductive pattern is formed (Japanese Patent Publication No. 43821/1973; corresponding U.S. Pat. No. 3,563,734 and DE OS No. 1797187). This image formation method will hereinafter be referred to as the second method.
Alternatively, as shown in FIG. 1 of the accompanying drawings, a photosensitive layer 1 is provided on a transparent conductive substrate comprising a transparent conductive layer 2 and a transparent back-up member 3 backing up the conductive layer to make a photosensitive member, and a DC bias is applied between a developing sleeve 5 to which a conductive magnetic developer 6 is caused to uniformly adhere by a magnetic field and the substrate of the photosensitive member by a power source 4 to thereby form a uniform potential difference between the developer and the surface of the photosensitive member, and image exposure P is effected from the transparent conductive substrate side of the photosensitive member at a position whereat the conductive developer on the developing sleeve is directed to the surface of the photosensitive member, whereby a visualized image by the developer corresponding to the image pattern can be formed on the surface of the photosensitive member. This image formation method will hereinafter be referred to as the third method. In some cases, in the image bearing member of FIG. 1, an insulating layer for holding charges is provided on one side or both sides of the photoconductive layer 1.
The first method described hereinabove makes the apparatus therefor complicated and expensive but yet inferior in reliability because it necessitates the equipment of control means, a high voltage generator for corona discharge, etc. which are involved in the process of charging.fwdarw.exposure.fwdarw.development.
The second and third methods do not require the charging means which is required in the first method and thus, the apparatus therefor can be very simple as compared with the apparatus for the first method. However, where a toner image is to be formed by the third method or a method similar thereto, it is unavoidable in order to obtain a sufficient image density that fog is created. That is, in the third method, the process of causing conductive developer to adhere to the surface of an N type photosensitive member, as shown in FIG. 2 of the accompanying drawings, comprises the step of applying such a DC bias between developing sleeves 5 each having a magnet therein and the transparent conductive layer 2 from a power source 4 that the sleeve side becomes positive. In this condition, conductive magnetic developer 6 is directed to the surface of the photosensitive member by a magnetic force and a uniform potential difference is formed between this developer and the surface of the photosensitive member while, at the same time, image light is applied to the transparent conductive substrate side. In accordance with photocarriers 8 created thereby in the photosensitive member 1 correspondingly to the light image pattern and having moved to the developer side, positive charges 7 are induced in the conductive developer 6 held on the sleeves. The developer 6 is caused to adhere to the surface of the photosensitive member by a coulomb force, as shown in FIG. 3 of the accompanying drawings. In this case, even in the dark portion to which the light is not applied, charge 9 on the developer 6 side and charges 10 on the transparent conductive layer 2 side are already induced by the capacitance of the area including the dark portion photosensitive layer between the conductive developer and the transparent conductive substrate and a coulomb force is acting therebetween, and by this force, the developer is caused to adhere to the photosensitive member even in the dark portion thereof and thus, fog in created.
In the foregoing, an N type photosensitive member has been taken as an example, but even if other photosensitive member is used, a similar result will be obtained by the DC bias applied between the sleeves and the conductive layer 2 of the transparent substrate. To solve the above-noted problem, it would occur to mind to reduce the DC bias applied between the sleeves and the substrate. By this, the absolute toner density of the fogged portion can be made sufficiently low, but sufficient photocarriers cannot be produced in the portion of the photosensitive member which corresponds to the light portion of the light image pattern and as a result, the amount of developer to adhere to the light portion of the light image pattern is deficient and thus, a sufficient density cannot be obtained.
The above-described second method utilizes the nature that it is difficult for conductive developer to adhere to the durable conductive surface formed on the surface of the photosensitive member by application of light thereto. However, the relatively small amount of developer having adhered to the conductive surface, like the fog described in connection with the third method, adheres to the surface of the photosensitive member due to a coulomb force equal to or less than the coulomb force acting between the developer and the conductive substrate and provides fog.
Accordingly, the main reason why the second and third methods or methods similar thereto are not widely used as a method for forming a toner image on the surface of a photosensitive member in spite of their having an advantage over the first method is chiefly the fog created by the principle as described hereinabove.